1. Field of the Invention
The present invention relates to an apparatus and process for improving the flexibility of operation of reactive distillation hydrogenation processes.
2. Related Art
The use of catalysts in a distillation column to concurrently carry out chemical reactions and separate the reaction products has been practiced for some time. Surprisingly, this use of a catalytic distillation column reactor lends itself particularly well for hydrogenations. See for example, U.S. Pat. Nos. 5,595,634; 5,599,997; 5,628,880; 5,773,670 and European Patent No. 0556025 B1. The combination is useful because the reactants in the liquid phase are quickly separated from the reaction products due to boiling point differences by fractional distillation. Thus further reaction is suppressed.
Several different arrangements have been disclosed to achieve the desired result. For example, British Patents 2,096,603 and 2,096,604 disclose placing the catalyst on conventional trays within a distillation column. A series of U.S. patents, including those listed above and more, particularly U.S. Pat. Nos. 4,443,559 and 4,215,011 disclose using the catalyst as part of the packing in a packed distillation column. The use of multiple beds in a reaction distillation tower is also known and illustrated, for example, in U.S. Pat. Nos. 4,950,834; 5,321,163; and 5,595,634.
In reactive distillations, such as catalytic distillation, as in any other distillation, there is no rigid cut off between the components. Reactions carried on in specified portions of the column using some constituents may leave undone other desirable treatment of other portions of the column constituents.
For example, mixed refinery streams often contain a broad spectrum of olefinic compounds. This is especially true of products from either catalytic cracking or thermal cracking processes (pyrolysis gas). These unsaturated compounds comprise ethylene, acetylene, propylene, propadiene, methyl acetylene, butenes, butadiene, amylenes, hexenes, etc. Many of these compounds are valuable especially as feed stocks for chemical products. Olefins having more than one double bond and the acetylenic compounds (having a triple bond) have lesser uses and are detrimental to many of the chemical processes in which the single double bond compounds are used, for example, polymerization. Sulfur and nitrogen compounds, among others, are frequently desirably removed also and they may be effectively removed from a portion of the column constituents, but because of different boiling points for other portions of the column constituents and the contaminants therein, not all of the contaminants may be removed.
Generally it is more difficult to remove both dienes and olefins than dienes alone. Diene-rich streams will hydrogenate at a higher volumetric rate under milder conditions than will a diene depleted olefinic stream. Sulfur in the several hundred ppm range is not uncommon for some feeds. Palladium hydrogenation catalysts are not able to handle such high sulfur levels, however, double-digit diene levels often present in these feeds overwhelm the sulfur impurities in their mutual competition for catalyst sites thereby providing reasonable rates notwithstanding.
In hydrotreating streams with high concentrations of dienes present (above 1000 ppm), there is a need to refrain from using high temperatures to avoid oligomerization. Generally, temperatures in the area of 170xc2x0 F. or above are avoided. Such operating restraints create conditions which are unfavorable for exhaustive olefin conversion in the same unit in which the diene is eliminated.
The present invention provides apparatus and process to address the reactive distillation hydrogenation of feed streams having concentrations of both mono and di-olefins.
The present invention includes an apparatus for conducting reactive distillations comprising a first distillation column, a first primary catalyst bed for carrying out a hydrogenation of unsaturated compounds comprising diolefins, said first primary catalyst bed being positioned in said distillation column to provide a first reaction zone for diolefins in said first distillation column, and optionally, a first secondary catalyst bed above said first primary catalyst bed, said first secondary catalyst bed to provide a second reaction zone for diolefins remaining in said first distillation column after said first reaction zone, a first mixed saturated/unsaturated compound feed entry below said first primary bed, a hydrogen feed below said primary bed, a bottoms line and an overhead line connecting to a second distillation column comprising a second primary catalyst bed for carrying out hydrogenation of unsaturated compounds comprising mono olefins from said first distillation column, said second primary catalyst bed being positioned in said distillation column to provide a first reaction zone for unsaturated compounds in said second distillation column, and optionally, a second secondary catalyst bed below said second primary bed, said second secondary catalyst bed to provide a second reaction zone for mono olefins remaining in the second distillation column after said first reaction zone, said overhead line from said first distillation column connecting to said second distillation column above said second primary catalyst bed and a hydrogen feed below said second primary bed.
The process carried out in the apparatus is also part of the present invention.
There may be distillation structures or trays between the primary and secondary beds. Hydrogenation reactions liberate a significant heat of reaction (on the order of 50,000 or greater BTU/lb mole H2 consumed). This released heat adds to vapor load in the column. Optionally, side condensers may be used to keep the uniformity of the vapor profile in the column within desired ranges. A secondary catalyst bed may be positioned in the distillation column, above or below the primary bed as heretofore described, to allow lighter or heavier boiling components to be exposed to additional catalyst and be purified or treated further.
The term xe2x80x9creactive distillationxe2x80x9d is used to describe the concurrent reaction and fractionation in a column. For the purposes of the present invention, the term xe2x80x9ccatalytic distillationxe2x80x9d includes reactive distillation and any other process of concurrent reaction and fractional distillation in a column regardless of the designation applied thereto.
The catalyst beds as used in the present invention may be described as fixed, meaning positioned in a fixed area of the column and include expanded beds and ebulating beds of catalysts. The catalysts in the beds may all be the same or different so long as they carry out the function of hydrogenation as described. Catalysts prepared as distillation structures are particularly useful in the present invention.